The present invention relates to the lighting arts. It especially relates to high intensity light emitting diode packages, components, apparatuses, and so forth, and will be described with particular reference thereto. However, the invention will also find application in conjunction with other solid state light emitters such as vertical cavity surface emitting lasers.
High power light emitting diode packages employ one or more light emitting diode devices operating at relatively high current levels to produce high brightness or high light output intensities. A light emitting diode chip has a voltage drop which typically is determined largely by the band gap of the material. The operating voltage of a light emitting diode chip is typically about 4 volts or less. Thus, generating high light output intensities involves increasing the operating current. A high operating current, in turn, leads to high resistive losses in cladding layers, electrodes, wire bonds, printed circuit traces, or other electrically resistive elements in the current path of the light emitting diode package.
These resistive losses translate into substantial heating of the light emitting diode package when operated at high power levels. Heating can produce thermal degradation of the light emitting diode chip, the chip electrodes, sealing encapsulant, solder bumps, or other components of the light emitting diode package. Moreover, heating generally increases the resistance of the electrical pathways and can reduce the light emitting diode operating efficiency. As a consequence, the light output power increase is proportionally smaller than the input electrical power increase.
Various thermal management techniques have been employed in light emitting diode packages. Encapsulating epoxies, sub-mounts, and the like are selected to provide high thermal conductivity to promote heat transfer away from the operating light emitting diode chip. Heat sinks are provided to collect and dissipate the generated heat. Chip electrodes are laterally distributed across the chip to provide current and heat distribution. Encapsulants and other thermally sensitive materials are chosen for good thermal stability and robustness. These design techniques reduce, but do not eliminate, thermal concerns in high brightness light emitting diode packages.
In some light emitting diode apparatuses, one or more tubular heat pipes are used to transfer heat away from the light emitting diode chips. Heat pipes include a heat transfer fluid, such as water, that undergoes a condensation/evaporation cycle to provide efficient heat transfer. In this cycle, the liquid evaporates in a hotter region of the heat pipe, absorbing heat during the evaporation. The gas phase material flows into a cooler region of the heat pipe where it condenses back into liquid form, releasing the absorbed heat during the condensation. A wick such as a groove, wire mesh, metal powder, or fibrous structure, is sometimes provided to promote return of the condensed liquid to the hotter region by capillary or wicking action. A condenser may also be provided in the cooler region to promote condensation.
Heat pipes have found some application in light emitting diode devices. For example, Board et al., GB 2,387,025, discloses a tubular heat pipe arranged to carry heat away from a light emitting device disposed at one end of the tubular heat pipe. A Peltier thermoelectric device is disposed between the light emitting device and the heat pipe to improve thermal coupling therebetween.
The arrangement of Board et al. is not well suited for distributing heat across an array of light emitting diode devices. In such an array, thermal management issues include not only removal of heat from the array, but also providing substantially uniform distribution of heat across the array. The use of the Peltier thermoelectric device for thermal coupling of the light emitting diode device with the heat sink is also problematic, since it increases the complexity of the light emitting apparatus.
Okino et al., U.S. Pat. No. 6,661,544, disclose a tubular heat pipe curved in a planar serpentine pattern and disposed beneath a planar array of light emitting diode dice or chips. Such a planar serpentine heat pipe can be expected to improve thermal uniformity across the array. However, “hot spots” may still exist in regions between the serpentine legs. These hot spots can be problematic if some chips are disposed over a leg of the serpentine heat pipe, while other chips are disposed between the legs. Moreover, Okino et al. uses an active heat pipe in which liquid is pumped or otherwise actively flowed through the serpentine heat pipe. The liquid flow can be expected to introduce thermal non-uniformity between the inlet and outlet ends of the planar serpentine tubular heat pipe. Actively driven liquid flow also requires a fluid source and drain, or a closed-loop continuous liquid pumping system, which is unsuitable for many light emitting diode apparatuses and lighting applications.
The present invention contemplates improved apparatuses and methods that overcomes the above-mentioned limitations and others.